Fix #1759: Hovering over an icon can trigger the scroll bar

[hwoongkang]

As @justvanrossum said: #1759 (comment), animating the svg inside the icon-button is indeed a good solution.

I changed the animating DOM object from the button itself to the svg inside the button.

I also added will-change: transform to the svg, in order to not to repeat the issue #1388.

Changing the animating object only didn't work in Windows. I added contain: content to the button, to indicate that the transform of its content should not affect the layout: https://developer.mozilla.org/en-US/docs/Web/CSS/contain

This fixes #1759

[justvanrossum]

Thank you so much! Will review and test soon.

[justvanrossum]

I tested on Windows: I managed to reproduce the original report and can confirm that your fix is working. Thank you so much!